Conventionally, a heat-treatment method is known which hardens only a surface layer of a steel material made of an iron-based metal, maintains toughness of the inside of the steel material, and imparts wear resistance and fatigue resistance. Examples of such a surface hardening method include quenching, a carburizing method, and a nitriding method.
As a surface hardening method for a ring-shaped member made of a steel material, quenching using a heating coil is used, for example. Specifically, a quenching device is known which heats a ring-shaped member using one or two heating coils, and then cools and quenches the ring-shaped member. For example, in the following Patent Literature 1, a high-frequency quenching device is disclosed which forms a uniform quench-hardened layer over the entire circumference of the peripheral groove of a workpiece by heating a portion to be heated of the workpiece using two high-frequency induction heating coils and cooling the portion. In addition, for example, in Patent Literature 2, a high-frequency quenching method is disclosed which heats the outer circumference of a ring workpiece using two or more (even number) movable high-frequency inductors, cools the ring workpiece by jetting a cooling liquid through coolant jetting holes provided at the high-frequency inductors, and thus quenches the ring workpiece.